It was 2009, and every time Adam St. Pierre joined the Boulder Trail Runners for a group run it felt like a race. An arms race, actually. A barefoot arms race.

"This was right after 'Born to Run,' and it was like an arms race to see who could wear the most minimal shoe," says St. Pierre, a gait-analysis biomechanist, exercise physiologist and coach at the Boulder Center for Sports Medicine. "Someone would show up in racing flats one day, then someone would say 'I'm wearing minimalist shoes,' then someone would be completely barefoot."

"Born to Run" is Christopher McDougall's runaway 2009 bestseller about ultra-running, which cited evolutionary biology and the Tarahumara Indians of Mexico in making a case that humanity is built for distance running -- and doing it barefoot. The book argues that running sans shoes with heavy, padded heels discourages over-striding and heel striking, thereby encouraging a more natural gait.

The Vibram Five Fingers, the most noticeable -- with individual pockets for each toe, they look vaguely amphibian -- and one of the earliest "minimalist" shoes, stripped running shoes down to a tough, thin rubber sole to protect from sharp objects. No support. No cushioning. Strange looking as they were, they took off.

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Three years later, the Five Fingers fad seems to be tailing off and the barefoot/minimalist trend has matured, leaving what many say is a permanent, and positive, mark on the running-shoe industry. Most notably, shoe manufacturers seem to have taken note that lower heel-toe height differentials -- some call it the "heel delta" -- may encourage a more natural gait, in which runners' feet land beneath their hips rather than out in front of their center of gravity.

But "(l)earning how to run and walk barefoot has been THE single thing that has transformed my body," says Josh Gordon, a Boulder physicist, who's been running barefoot since 2010. He no longer experiences the back and hip pain that used to interfere with his running. "I finally feel, well ... athletic."

Many others, however, have experienced problems running barefoot or minimalist, and some medical practitioners say they've seen an uptick in problems caused by the trend.

Amy Thoe, who lives in Denver and sometimes runs with the Boulder trail group, "did barefoot for awhile and (has) since moved on to (Boulder-based) Newton shoes which have a lower heel and additional padding for the balls of the feet. I found the transition to barefoot to be incredibly slow and painful and the payoff quite small."

Ditto for John Beckwith of Boulder, whose two ventures into barefoot resulted in a broken second metarsal. But even he credits minimalist running with teaching him a different stride that allows him to run more miles per week.

"I can run practically any distance without getting achy, sore, or any of the other irritating injuries that used to crop up," he says.

Despite the trend, going barefoot/minimalist isn't for everyone. Foot physiology, running patterns and other factors should be considered before making a switch.

"It's like a carbon-fiber mountain bike frame, which are certainly advantageous from a performance standpoint," says sports chiropractor Jeremy Rodgers of Louisville. "But they are not for the entry-level rider who doesn't yet have outstanding riding mechanics and still rides with more guts than grace. Minimalist shoes are likewise not for the entry-level runner."

"It's great for a lot of different things, to strengthen your feet and cross-training," says Johnny Halberstadt, co-owner of the Boulder Running Company, which recently narrowed its selection of the Vibram shoes. "But it's a quaint notion that 'Hey, there has been a ripoff by the shoe companies, shoes have been terrible, and all you've got to do now is take off your shoes and you'll be much better off as a runner.'"

In fact, Vibram was sued in March for claiming that its shoes provide "health benefits" that accrue to neither true barefoot running nor "traditional running shoes" with more cushioning stability features. The suit has yet to be resolved.

As Halberstadt's business partner and former world-champion marathoner Mark Plaatjes predicted in 2009, many runners who ran with the minimalist trend have wound up with injuries: "The majority of people can't do this," he said at the time.

"Some people switched cold turkey (from more traditional shoes) and started having a lot of calf and Achilles problems," St. Pierre says, noting that he and his colleagues also have recommended flatter shoes to correct other problems in their clients.

But make no mistake, there is a dedicated community of barefoot/minimal runners -- including those who genuinely use no shoes -- out there who swear by their gear.

Some true barefooters -- some of whom even run unshod in winter -- argue that retailers and shoe companies have a vested interest in selling shoes, so it's no wonder they aren't happy with the trend.

"The barefoot running movement (true barefoot, not in a glove) hasn't gone away, nor have many converts" according to Michael Sandler, co-founder of the Boulder Barefoot Running Club. "New members keep joining."

Many therapists say problems aren't caused by the shoes (or lack of them), but lack of technique needed to run barefoot. And not everybody should be running barefoot or in minimalist shoes, Rogers says. He notes says the lower "delta" has reduced such problems as patellar tendonitis, but that the increased flexibility has caused a relatively small increase in "tension injuries" to the Achilles and plantar fasciitis on the bottom of the foot.

"In general, I support the concept of minimalist shoes for runners who take the time to implement them on a conservative timeline and learn outstanding running mechanics, otherwise known as active motion control methods," he says.

The science, so far, indicates that placement is more important than whether a runner is a heel striker or mid-foot or forefoot striker, at least in terms of energy expended by runners. Running in very lightweight shoes surpasses running barefoot or even minimalist, according to research by Rodger Kram of the Locomotion Laboratory of the University of Colorado, Boulder, according to an Oct. 15 blog post in The New York Times. And, the research found, some cushioning -- 10 millimeters, to be exact -- actually improves performance.

"Those who extol (barefoot and minimalist running) overlook three studies showing it is not more efficient," Kram told the Times. "Those studies showed striking midfoot or forefoot was no better and no worse than heel striking."

And, Halberstadt notes, two of the world's three top marathon runners, including Haile Gebreselassie race in the Adidas Adizero Adios, a relatively light, but cushioned "racing flat." He's also a heel striker who trains in heavier shoes, Halberstadt says.

"If you go out to the big marathons I don't think you'll see more than a small percentage of people" running barefoot or in minimalist shoes, he says. "The elephant in the room is performance. If they led to super fast times you'd see all the top runners in barefoot or super minimalist shoes."

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